The photoreceptor system that transduces light stimuli for neuro- endocrine, circadian, and therapeutic effects of light is unknown. The overall aim of this research is to determine the receptor mechanism that mediates these biological and therapeutic effects of light. To achieve this aim, an action spectrum (the relative effectiveness of different wave lengths for eliciting a biological response) will be established to help identify the photoreceptor system for light regulation of the hormone melatonin in humans. This action spectrum may then be used as a tool for investigating the action spectrum and related photoreceptor system involved in circadian regulation and light therapy. The specific aim of the proposed research is to determine an action spectrum for light-induced plasma melatonin suppression using 8 monochromatic wavelengths (10mm half-peak bandwidths) between 400 to 760 NM. The effects of these wavelengths on the acute suppression of melatonin will be studied in normal volunteers. fluence-response curves covering a 5 log unit photon density range from 10<14> to 10<19> photons/cm<2> at the level of the corneal will be established for each wavelength. an action spectrum will be constructed by plotting the reciprocal of the half-saturation stimulus (sigma<-1>) from each of the 8 wavelengths tested. This action spectrum will clarify the range and peak sensitivity of the receptor system that transmits photic stimuli to the human neuroendocrine and circadian system.